This invention relates to nuclear magnetic resonance (NMR) imaging systems and, in particular, to techniques for acquiring NMR information signals with short and long spin echo delays.
In NMR imaging systems there are two general techniques for collecting NMR information signals. One is to detect the free induction decay (FID) signals which are present immediately following a radio frequency (RF) excitation pulse. The immediacy of the occurrence of these signals is advantageous is that it allows NMR information to be gathered quickly, which can shorten the time required to gather the desired information. However, their near proximity in time to the excitation pulses makes detection difficult, since residual effects of these pulses can interfere with accurate detection of the FID signals. Also, switching from RF pulse transmission to FID signal reception becomes critical, especially when a common RF coil is used for both transmission and reception.
A second technique for NMR information acquisition is the spin echo technique. In this technique, the FID signals following the RF excitation pulses are ignored. At a later time a 180 degree inverting RF pulse is applied to refocus the previously excited spins. After another time interval a spin echo signal is produced which resembles back-to-back FID signals. The spin echo signal may be detected at a time when RF pulse effects have substantially dissipated, making data collection less critical. Also, in the time interval between the initial RF excitation pulse and the resulting spin echo signal, numerous gradient fields can be applied to the subject being imaged to imbue the spin echo signals with greater information content.
In the prior art, it is known that successive inverting RF pulses may be applied following the initial excitation pulse to repetitively refocus the spins. This results in the production of a sequence of spin echo signals. It has been found that spin echo imaging experiments which gather sequences of NMR information in this manner can produce at least two images of differing diagnostic utility. Images constructed from spin echoes occurring earlier in time relative to the initial excitation pulses will appear crisp and sharp. This is due to the high signal to noise ratios of these signals. When used for medical diagnosis these images will provide good anatomical representation. However, signal contrast will be relatively low.
Images constructed from spin echoes occurring later in time will have different diagnostic utility. Because of their relatively poorer signal to noise ratios, these images can appear to be smudgy or noisy. However, these images can exhibit relatively high spin-spin relaxation contrast. which provides good tissue characterization.
In the spin echo sequence techniques of the prior art, the successively occurring inverting RF pulses are applied at precisely timed, equal intervals after the RF excitation pulse. This results in the production of spin echo signals at regularly recurring time intervals, affording ease in timed signal acquisition. However, the present inventors have found that these techniques result in a complex form of harmonic signal distortion. Each regularly recurring spin echo signal will contain signal artifacts which are harmonically-related reflections of earlier applied and produced signals. By the time that the later occurring spin echoes are produced, these signals will contain artifact components resulting from amy imperfections in the earlier applied RF pulses. A great deal of the contrast of these later spin echoes can become obscured by these artifact components. Furthermore a larger number of RF pulses must be applied before a desired, later occurring spin echo signal is produced. It is desirable to reduce the amount of RF heating produced by these pulses by generating the later occurring spin echo signals with as few RF pulses as necessary in a given experiment.